1. Field of the Invention
The present invention relates to hydraulic pumps, and, more particularly, to positive displacement hydraulic pumps.
2. Description of the Related Art
The development of successful multi-cylinder axial and radial hydraulic pumps are numerous. The ability to develop high pressures in small lightweight units have made these pump designs ideal for use in the aircraft industry. Pressures in excess of 5000 psi are common. With variable swash plate and radial piston designs (such as used in hydrostatic transmissions), high pressures at zero flow are obtainable. Other applications include refrigeration compressors with cryogenic service.
Conventional axial and radial hydraulic pumps typically include a cylindrical piston which is disposed within a cylinder and connected to a piston rod. Such pumps are used to effect both fluid flow and power transmission. The piston rod either drives or is driven by the cylindrical piston to effect fluid flow or power transmission, respectively. Alternatively, the piston may be generally sphere-shaped and likewise connected to a piston rod. With such sphere-shaped pistons, the piston rod similarly either drives or is driven by the sphere-shaped piston to effect fluid flow or power transmission, respectively.
A recently developed compressor covered by U.S. Pat. No. 5,316,447 to Fuji et al. discloses a movable discharge valve actuated by rising pressure. Another self-reversing hydraulic control for use with a reversing hydraulic pump disclosed by U.S. Pat. No. 4,213,298 to Milgram uses sensors to control direction. A rotary fuel pump with opposed end faces, pistons, return springs, and an adjustable cam plate is taught by Barnard et al. in U.S. Pat. No. 3,216,367. These patents disclose a common theme of utilizing directed fluid to actuate reversing or changing of flow direction.
A problem with conventional axial and radial hydraulic pumps is that they are inherently relatively expensive. The pistons, piston rods, swash plates, pintles, fluid flow passages in the housing, etc. result in a relatively expensive pump which may be overpriced for certain applications.
Conventional hydraulic pumps may also be in the form of continuous flow pumps such as gear pumps. Such pumps typically include an internal gear which is disposed within and eccentric to a ring gear. Rotation of the internal gear and ring gear causes a continuous flow of fluid through the pump. A problem with continuous flow pumps is that a predetermined amount of displaced fluid cannot be accurately achieved.
One possible application for conventional multi-cylinder radial and axial hydraulic pumps is in the recreational vehicle industry to expand travel trailers, motor homes and tent campers by sliding, tipping, or raising portions thereof. However, although such pumps tend to be relatively quiet and reliable, they have not heretofore been cost effective for such applications. Other alternative power sources which have been utilized in an effort to reduce costs include electrically powered screws, hydraulic gear pumps, solenoid valves, double acting cylinders, cable and pulley equalizers, etc.
What is needed in the art is a hydraulic pump which is simple, provides reliable output power, and is relatively low cost.